Currently, the lighting technology utilizes a wide spectrum of light sources, such as light sources with filaments, halogen light sources, high-intensity discharge lamps (HID), fluorescent lamps and, lately, more and more often semi-conductor light sources, such as light emitting diodes (LED). The alternative to the currently introduced light sources of the LED type appears to be a laser diode (LD). The laser diode is, same as the LED, a semi-conductor device which converts electrical energy directly into light.
Currently, the output of the blue laser diodes as one of the types of solid-state source of coherent light progressed significantly and it is therefore possible to consider the utilization of laser in lighting technology. The semi-conductor laser achieves very high luminance which, currently, cannot be attained with any other known light source, with values more than 1000× greater in comparison with the conventional surface lighting LEDs. The luminance of the high efficiency LED light sources is 60 cd/mm2-100 cd/mm2 as opposed to blue lasers which are able to generate luminance exceeding 500 cd/mm2. Currently, blue laser diodes based on the InGaN technology are available on the market.
In case of high current density (˜kA/cm2), necessary to attain high luminescence intensity, lasers are the most efficient converters of electrical energy into optical energy. The laser diode produces monochromatic coherent light with high luminance (great energy density), narrow spectrum width and narrow emission characteristic which can thus be focused easily and excellently. The laser diode therefore represents an efficient source, providing a promising condition for a high density output light source.
Due to the high luminance produced by the laser diode on a small emission surface it is possible to achieve reduction of dimensions and weight and, subsequently, a larger design variability of light sources. The laser diodes therefore appear to be a logical successor of the light emitting diodes.
To obtain white light when utilizing a laser diode, it is possible to use the additive composition of the blue, green and red laser light beams. The disadvantages of this procedure consist of the fact that lighting requires a higher electrical energy consumption, that is by three laser diodes at the same time, and, furthermore, the light sources requires very exact optics for mixing the laser beams in one white light beam. The different thermal ageing of the individual diodes results in the changes of the resulting color spectrum during lifetime.
Presently, a laser diode begins to be combined with a phosphor which is a kind of luminescent material. The phosphor is any material that absorbs excitation light of a certain wavelength and emits it on a different wavelength in the visible spectrum of light wavelengths. This phenomenon occurs most frequently during the conversion of the short wavelength light into the longer wavelength light and is called “downconversion”.
The phosphor transforms the light that is being excited into the light of the required wavelength. When doing so, there are energy losses in the phosphor both due to the conversion itself (Stokes shift) and to the light scattering or reflection. When utilizing a laser diode, it is expected that it will be necessary to ensure the heat dissipation from the phosphor of the ˜1W output. Based on the requirements of the given application, more laser diodes can be utilized for phosphor excitation, however, with increased demands for phosphor and diodes assembly cooling.
The light beam, produced by the laser diode, is characterized by high energy density, narrow profile and high directionality, therefore, new requirements are placed on the phosphor. It is thus necessary to use the phosphor in such a form that can withstand high loading without damaging or degradation. One solution is to utilize a rotational phosphor to spread the incident power onto a larger surface, as it is, for example, with the X-ray radiation sources or as stated in the WO2012172672A patent application. The rotating parts are, however, generally more demanding for construction, maintenance and, particularly, operation and are a potential source of defects. This solution is not suitable for the general application and therefore a static solution must be found.
When illuminating phosphors, assembled in various matrixes, with a laser light beam of the 1 W output, the matrixes based on polycarbonates, glass or aluminium are damaged after 5 minutes due to the released heat. A ceramic matrix can withstand the thermal conditions. However, its temperature can exceed 300° C. The final temperature, to which the phosphor is heated during the conversion, depends too on the volume of the used phosphor, since its decreasing volume results in a non-negligible increase in temperature. Under high temperatures, there occur negative phenomena too, such as temperature luminescence quenching, and the whole light source can be overheated which furthermore increases the demands on cooling.
One of the disadvantages of utilizing laser diodes is due to the fact that with laser diodes there is a large thermal dependence since the increase in the temperature on the laser diode causes a significant decrease in its efficiency and shortening of its life. Thus it is necessary to eliminate the supply of other heat generated in the phosphor back to the laser diode chip. One of the available solutions is to utilize a “remote phosphor” (as described in the patent files WO2010/143086A1; EP2202444; WO2009/134433A3) which is used in many construction assemblies. Thanks to the physical separation of the excitation source and the phosphor it is possible to control the heat management well. The solution to the heat dissipation from the phosphor is described e.g. in the patent application US20110280033. When physically separating the source of the excitation light and the phosphor, it is possible to conduct the excitation light from the laser diode to the phosphor with optical fibers and it is possible to use preferably the small emitting area and a high directionality of the beam generated by the laser diode for simple coupling of the beam in the optic fiber.
Another alternative is to utilize the phosphor in the form of a single crystal. The single crystal represents a highly arranged, perfect material where atoms are located in lattice positions. Due to this fact, the light scattering is minimal in the single crystal phosphor. In the single crystal material, the doping atoms of the Ce chemical element are always distributed in the position in which they act as efficient luminescence centers.
The amount of the blue light, absorbed and subsequently converted by the phosphor, is directly proportionate to the concentration of the Ce3+ doping ion. For this reason the concentration of the Ce3+ doping ions is deliberately increased in the conventional powder phosphors. This results in higher heat generation in the phosphor which, if it is not efficiently taken away, may heat up the phosphor to temperatures exceeding 200° C. and there may occur temperature quenching of the luminescence which denotes thermally dependent non-radiant processes that decrease luminescent efficiency. Nevertheless, the temperature quenching with a single crystal phosphor on the basis of YAG:Ce occurs only above the temperature of ˜350° C.
Due to the perfect atoms arrangement in the crystal lattice, the single crystal phosphor achieves high thermal conductivity. The dissipation of the heat, generated during luminescence, thus shall be more effective than with current powder phosphors, spread in silica gel, glass matrix (PiG—phosphor-in-glass) or in a matrix in the form of a polycrystalline structure.
Because of the absence of grain boundaries and minimum of defects, contained in the single crystal, there is only small scattering of the generated heat. It is therefore possible to use a lower concentration of dopant or dopants which, subsequently, reduces the phenomenon called concentration quenching of luminescence, responsible for the reduction of luminescence efficiency. The lower concentration of the Ce3+ dopant in the single crystal phosphor results too in the lower amount of heat, generated during luminescence with the Stokes shift and the reduction of thermal loading. At the same time, if another element which has a different atom diameter than the original is deliberately introduced in the phosphor crystal lattice, there will occur expansion or distortion of the crystal lattice and, subsequently, the shift of the phosphor emission spectrum.
When using a laser diode as the excitation source, it is necessary to ensure its safe use (“eye safety”). This can be attained either by using a sufficient volume of the single crystal in such a manner that there is sufficient conversion of the laser beam, or by using a reflective element located behind the phosphor in the direction of the laser diode beam so that it should reflect the radiation passed through the phosphor back to the phosphor where it will be fully absorbed and converted.
The WO 2012/170266 patent application utilizes a phosphor on the basis of YAG:Ce where a portion of the Al atoms may be replaced with atoms of the Ga chemical element and a portion of the Y atoms replaced with Ce atoms. A solid-state lighting device is used as the excitation source, which can be a LED or a laser diode. The disadvantages of the above described solution consist of the fact that their absorption and emission spectra cannot be shifted in such a manner that the resulting light meets the conditions for non-disturbing long-term lighting of e.g. households.
The PV 2013-301 patent application deals with the application of the YAG, LuYAG and GGAG single crystal phosphors into the light sources which are doped with Ce, Ti, Cr, Eu, Sm, B, C, Gd and Ga and which are excited in the light source with a blue LED. The excitation source from the light emitting diodes does not attain the above stated quality of the laser diode light beam, therefore the light sources produced in this way are not very powerful.
In the WO 2009/126272 patent application, a single crystal phosphor on the basis of YAG is similarly used and it emits in the yellow, green, orange or red spectrum part in combination with the light emitting diodes. The disadvantages of this solution again consist in LEDs being used as an excitation light source.
In the patent application number US20080283864A, there is used as a phosphor in the solid-state light emitting device a single crystal material, composed of Y3Al5O12 doped with Ce or Eu, or CaxSryMg1-x-yAlSiN3 doped with Ce, or SrxGaySz doped with Ce, or Sr2-xBaxSiO4 doped with Eu2+ (BOSE), or Eu2+ doped single crystal from the group CaxSr1-xAlSiN3, SrxGaySz, α-SiAlON, siliceous garnet, Y2O2S and La2O2S. The content of Ce ranges between 0.1 to 20% and of Eu between 0.5 to 20%. The disadvantages of the stated solution consist in the fact that due to the chemical composition of the single crystal phosphor it is not possible to shift the limits of the absorption and emission spectra of this single crystal phosphor. The extracted light has unchangeable parameters and must be possibly combined with light from a different light source.
In the US20040200964A1 patent application, there are presented single crystal materials CexLu(1-x-z)AzAl(1-y)ByO3, where x=0.00005−0.2, y=0.00005−1, z=0−(1−x) and A is one or more from cations Y, Sc, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, In, Ga; B is one or both cations Sc and Ga; and material CexLu(1-x-z)Az Al(1-y)ByO3, where x=0.00005 to 0.2, y=0.0 to 1.0, z=0.0005 to (1−x) and A is one or more from cations Sc, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, In and B is one or both cations Sc and Ga, furthermore A contains one or two cations Y and Ga in the application of the scintillator which means utilizing its scintillation after excitation by high-energy radiation, such as X-ray, gamma or beta. The disadvantages of the stated single crystal material consist in its not being suitable for light sources for general usage, e.g. in households. The used excitation radiation is harmful to the health and is not suitable for common light application.
Polycrystalline phosphors, useable for light sources according to the current state of technology, have the following disadvantages which consist in the utilization of unsuitable excitation radiation, in their polycrystalline structure which leads to energy losses, in complicated realization of rotational phosphors and in worse heat management, leading to quenching and damage of radiation sources. Furthermore, the excitation radiation source is either harmful to the health, that is gamma, X-ray and UV excitation radiation, or the excitation radiation source is not sufficiently efficient as in case of e.g. LED excitation radiation sources whose light beam luminance is low and the light beam is divergent.
The known single crystal phosphors combined with excitation radiation laser sources are of such a construction that they do not enable the shift of the emission and absorption spectra, therefore the extracted light must be further combined and, as a result, such light sources are bigger and more costly.
The task of the invention is to remove the above stated drawbacks of the current solutions and to create a light source that would use a solid-state laser source of excitation radiation that would be more effective when converting electrical energy into excitation light radiation, that would include a manufactured single crystal phosphor, that would radiate extracted light of a pleasant color for long-term utilization e.g. in households and that could be well applied in various areas of human activity, necessitating various technical solution of light sources. The light source could be minimized, it would have low production costs, the extracted light would be bright enough and the light source would have no problems with heat management.